Visit 3: heterophoria: in the first of a two-part series on heterophoria, optometrist Amar-Kaash Gandecha discusses its identification and investigation.
Firstly, it is of paramount importance to distinguish between a compensated phoria (which is normal), and a symptomatic, decompensated phoria. Using the alternating cover test, a latent misalignment of the visual axes (phoria) will be detectable since the eye under the cover reverts to its natural resting position, owing to a lack of visual stimulus. Most people will be able to overcome these phorias with little difficulty, due to fusional reserves, bringing the eyes into alignment on the fixation target for binocular single vision. However, when someone is unable to maintain this alignment (compensate for the phoria), it becomes decompensated and will likely become symptomatic.
Symptoms of decompensated heterophoria may be the main reason that a patient attends an eye exam; so identifying them can help you to tailor your investigations. Generally, symptoms are associated with some particular use of the eyes for prolonged periods and tend to increase as the day goes on. These can be grouped into visual problems (for example diplopia and blurred vision), binocular problems (for example closing poor depth perception) and asthenopia (for example headaches and aching eyes). Unfortunately, these symptoms are nonspecific and will require further evaluation.
The cover test is vital for assessing the status of heterophoria. You should note the direction of phoria (eso-, exo- or hyperphoria) and the speed of recovery.
A cover test should be performed with and without the refractive correction. This is particularly important in the presence of esophoria in young hyperopic patients, where the refractive correction may resolve the decompensation due to the accommodation-convergence relationship.
If the symptoms and cover test suggest a possible decompensated heterophoria, the next step would be to carry out the Mallett unit fixation disparity test.
Fixation disparity occurs when the object of fixation does not fall exactly on corresponding retinal points of the eyes, but instead falls within Panum's fusional area, which is a cortical phenomenon that prevents diplopia from occurring.
Where there is a decompensated heterphoria, the motor visual system is unable to maintain alignment of the eyes to allow the object to fall within Panum's area, leading to diplopia. In its truest terms, the Mallett unit will find the aligning prism, which represents the extent of the uncompensated part of the phoria. In esophoric deviations, base out prism is introduced in front of one eye, increasing until the nonius markers on the Mallet unit line up (use base in prism for exophoric deviations and base up for hyperphoric deviations). The Mallett unit is very useful for prescribing prism and modifying the spectacle prescription.
Fusional reserves are a measure of how much vergence the person has 'in reserve', in order to overcome a phoria. These are measured using a prism bar, and the fusional reserve opposing the phoria observed in the cover test should be measured first--for example base out to force convergence in exophoria. The patient fixates a target and the prism power is slowly increased until the patient reports blur and then break (that is diplopia). The power is then reduced until recovery (that is single vision) occurs. Results can be compared to normative values, or can be interpreted using Sheard's criterion (opposing fusional reserve should be twice the degree of phoria--best in exophoria) and Percival's criterion (the opposing fusional reserves should not be less than half the other--best in esophoria).
If the patient is not reporting any symptoms, but the cover test and Mallett unit indicate the need for an aligning prism, it is advisable to carry out the Mallett unit foveal suppression test. Suppression may be a compensatory mechanism to avoid symptoms; however, it reduces the efficiency of the binocular vision system.
The final article will focus on the management of decompensated heterophoria.
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|Title Annotation:||PRE-REG FOCUS|
|Date:||Feb 22, 2013|
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